A6 rear differential fluid change

Need your collective wisdom, BTDT type of stuff.
Patient is a '99 A6 Avant new to the household (120K miles). Wanted to change out or at least check level of the rear fluid but the fill plug is really solidly stuck in there. I tried perhaps 200 ft-lbs of torque with simple ratchets and no movement. Soaking in liquid wrench for a few hours and still no movement.
I was going to take the brute force method and apply an impact wrench to it but then thought that might crack the case and I certainly don't want to do that. Any other thoughts to get the fill plug out? Or have others applied an impact wrench with success?
--bob
'86 5Ks Avant
'99 A6 Avant
'02 TTQR
'07 Q7 4.2P